Many modern automatic transmissions use trim valves to control the pressure rise of apply pressure in an oncoming friction device such as a disc clutch or disc brake. The trim valve controls the apply pressure increase from a value sufficient to overcome the piston return springs to full engagement pressure. The maximum full engagement pressure occurs during vehicle operation requiring maximum engine torque and in some instances, maximum engine torque multiplied by the torque ratio of the torque converter which can be as high as 3.0.
The system requires that the pressure be controlled between 0 (zero) and 250 psi. This pressure range, in current transmissions, is generally a straight-line function. With the current range of pressure available from conventional trim valves, this requires a 3 to 1 pressure gain. For example, if the apply pressure is from 10 to 270 psi, the trim pressure is from 3 to 90 psi which is consistent with current valve technology. The trim pressure control at the lower end of the range is very important, since during this portion of the apply function, it is necessary to overcome the piston return springs and initiate friction engagement. If the pressure rise is too rapid, a harsh "shift feel" will occur.
With currently available trim valves, the pressure must be controlled in the range of 3 to 5 psi (9 to 15 psi apply pressure) to accommodate this function. Thus, extremely accurate controls are necessary. This situation is exacerbated by the need to provide the control of apply under a plurality of throttle conditions, including a full throttle maximum condition.